1. Field of the Invention
The present invention relates to an image processing method and apparatus for generating a pseudo three-dimensional image from three-dimensional image data of a group of multitudes of two-dimensional images by a volume rendering method. The invention also relates to a computer readable recording medium on which a program for causing a computer to perform the image processing method is recorded.
2. Description of the Related Art
In order to facilitate understanding of a three-dimensional structure and the like of a subject, a pseudo three-dimensional image generation and display process is performed, in which three dimensional image data of the subject constituted by a group of multitudes of two-dimensional images obtained by CT equipment, MRI machine, ultrasonic diagnostic equipment, and the like are stereoscopically visualized on a two-dimensional plane using computer graphics technology or the like.
As for the method of generating such pseudo three-dimensional image, a volume rendering method is known as described, for example, in Japanese Unexamined Patent Publication No, 2006-000338, in which an opacity level and R, G, B color information are allocated to each voxel constituting a three-dimensional image, and voxel tracing (ray casting) is performed from the observation side to each pixel on the projection plane, thereby an internal tissue structure of a subject, as well as the interface between tissues, is visualized through multiple layers.
In the volume rendering method, a “shading” process is performed in which an arbitrary light source is set, and a three-dimensional appearance is represented based on the relationship between an incident direction of light from the light source incident on an arbitrary point on a three-dimensional image illuminated by the light source and an inclination of a plane defined by a gradient of pixel values at the point. More specifically, a brightness value b(Pv) at an arbitrary point Pv on the three-dimensional image is calculated by multiplying color information c(Pv) allocated based on predefined color information with respect to each tissue (each input pixel value, such as hounsfield number) of a subject with a vector inner product between the normal vector N(Pv) to a plane defined by a gradient of pixel values at the point Pv and a unit vector L from the point Pv to the light source, as shown in formula (2) below.b(Pv)=h(N(Pv)·L)×c(Pv)  (2)
where h is the shading coefficient by diffusion reflection.
In the conventional shading process described above, however, attenuation of illuminance caused by the blockage of light emitted from the light source toward each point on a three-dimensional image by any other subject image is not considered. Consequently, as shown in FIG. 6A, in a case where a portion of light emitted from a light source S for illuminating a subject B is blocked by a subject A and a shadow of the subject A should appear on the subject B, for example, a pseudo three-dimensional image I2 generated by the volume rendering gives rise to a problem that the image is unnatural since such shadow is not rendered, as shown in FIG. 6B.
In view of the circumstances described above, it is an object of the present invention to provide an image processing method and apparatus capable of improving the fidelity of a pseudo three-dimensional image generated by the volume rendering method. It is a further object of the present invention to provide a computer readable recording medium on which a program therefor is recorded.